Optical units which are provided with an optical system by which a front object and an approximately lateral object can be simultaneously observed and a lens barrel that holds the optical system have been known up to now. In this case, the range which the above term, “approximately lateral”, defines includes not only the lateral side of the optical system itself but also the diagonally forward and diagonally backward sides of the optical system.
In optical systems for such optical units, an optical system which is formed in such a way that, after light from the approximately-lateral-object side is reflected two times on the inside, the light emerges to the image side to be guided to an imaging element like CCD (Charge Coupled Device), CMOS (Complementary Metal-Oxide Semiconductor), or the like has been known (for example, refer to International Publication No. 2003/042743).